OK Kelvin, if these aren't interference patterns, what are they?
My brother is a physics professor, who teaches optics. He said years ago that I couldn't be seeing interference patterns on a macroscopic scale, from light coming through a narrow crack in a door hinge.
Here, the light is coming sideways through a narrow gap between a curtain and window frame. The bands certainly look like interference patterns to me. (When I was an oceanography student, I used to watch the waves reflecting off the sides of the ship canal as I walked to school. They made interference patterns as they crossed. I also knew all the types of wave patterns I could get in a coffee cup from my hand shaking as I walked across the cafeteria.)
This is a South-facing window, and the photo was taken on 2-28-2007. It has been photoshopped only to bring out the best in the image. I think that if the light is coming sideways to the opening, you can get the effect of a very narrow slit.
Or, if these aren't interference patterns, what are they?